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Homework Help: Calculating Torque and Tangential force from X,Y Vectors

  1. Jan 16, 2005 #1
    Hello all,

    I'm have a software program I using which gives me an output the X and Y vector forces at Point P on a disk, and I was wondering what calculations I need to use to convert the forces on Point P to the angular (Tangential?) force on the edge of the disk; I know the position of point P, as well as the size of the disk. I was also wondering how to calculate the amount of torque on the disk from the Vector forces of point P. I drew up a small diagram to help illustrate what I'm trying to do. Any help will be greatly appreciated.

    Jason O

    Attached Files:

  2. jcsd
  3. Nov 25, 2006 #2
    sorry dear no idea for this because i am very bad at maths.
    but still i will try to get the answer for this bye.
  4. Nov 26, 2006 #3


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    I am assuming P is an arbitrary point on the disk. There will be a vector r from the center of the disk to point P. That vector will have components x and y so in terms of the usual horizontal and vertical axes you have

    r = x i + y j
    F = F_x i + F_y j

    The vector r forms an angle θ with the positive x-axis. In terms of this angle x = r*cosθ and y = r*sinθ. At P the vector F forms an angle φ with the positve x axis. In terms of this angle F_x = F*cosφ and F_y = Fsinφ. The equations for r and F become

    r = r*cosθ[ B]i[/B] + r*sinθ j
    F = F*cosφ i + F*sinφ j

    What you are looking for is a way to express F in terms of components in the direction of r and in the perpendicular direction. If you move P to some other location in your diagram and draw the vectors r and F you should see that there is an angle between r and F that can be expressed in terms of θ and φ. But the force stays at point P. It is not applied at the edge of the wheel. Even if you leave the vectors in terms of x and y components, you can compute the torque from the defining equation

    τ = r x F
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